System won't boot: Graphics card issue, or PSU issue?

All of a sudden, for some reason, my PC won't boot with the 6-pin connectors plugged into my XFX HD6870 1GB. But when I unplug them, my system boots/starts up and I can hear the fan going on the graphics card at 100%.

This is the first time I've ever come across this, so my question is. Is this a PSU issue, or graphics card issue?

I'm trying to find a spare video card to test out, but I'm really confused by this. Thanks in advance.

have you try another 6 pin cable ??? cause i don't think no power on that card is cool, it will damage your card for sure... that's why the fan is 100% running... it's seems that your psu triggered on overcurrent cause it's not capable of running that much amp on 12v...

@spawnkiller - I don't have any 6-pins spare (the PSU only has two, the HD6870 requires two). I've been running this card for just over 2 months. It's a WinPower Plus 800W, a rather obscure brand (which I've been using for around 2 years).

@david179057 - Yeah.

Like I said, this was random. I was using it for several hours before hand too, just needed to reboot after wanting to do a standard bootscan with "Avast!".

Edit:
System is running fine with my older VTX HD6770. Sent off the XFX HD6870 for RMA since it's still under warranty - hope to get a replacement.

Ok, if you have a chance, try your PSU with a bigger card to see if it's capable of running this amp... PSU degrade over time so 2 years after maybe it's only delivering 500w now...

PS: if you have 3-4 12V rail, maybe try to keep the GPU alone on 2 of them to be sure to have enough power...
My Antec NeoHE 500w had powered a GTX560 no problem but i had to plug it into it's own 12V rail to running it no problem (when plugging fan and HDD, sometimes crash and voltage monitor of my board indicate 11.56V on 12V so it was not capable of running that, however it has like 7-8 years old so it's normal)

Agreed, PSU degrade over time can cause this. Also, a burned (or corroded) cable can, as well. You may want to break out a multimeter and check voltage throughput.

Secondly, a cooked GPU can cause this, as well. With power connected, it creates a fault and the system fails due to the PCI-e bus locking. Power disconnected, the GPU aside from the basic renderer doesn't connect and the slot passes into inactive.